Different types of control systems for hitch assemblies on work vehicles such as agricultural tractors are known. The purpose of many hitch control systems is to control the elevation of the hitch assembly to maintain a substantially constant depth of ground penetration for the ground-penetrating implement (e.g., a plow) hitched to the hitch assembly, or to maintain a substantially constant draft force exerted on the hitch assembly by the hitched implement's interaction with the ground, or to maintain a desired combination of position and draft force control. The control systems provide control signals to hydraulic valves which control the flow of pressurized hydraulic fluid to a hydraulic actuator coupled to the hitch assembly, wherein the hydraulic actuator responds by raising and lowering the hitch assembly, thereby raising and lowering the implement hitched thereto.
Known hitch control systems include simple control devices for generating a position command or a draft force command for the hitch assembly and, thus, for the ground-penetrating implement. The controls typically include a position command device for setting a desired position command, and a draft force command device for setting a desired draft force command. Some systems further include a "mix" control for mixing the position and draft force commands. Known systems also include position and load sensors for providing position and draft force feedback signals. To maintain the desired position or draft force, changing soil conditions in the field being worked create a need for raising and lowering the hitch assembly in closed-loop fashion in response to the respective feedback signal.
An exemplary hitch control system of the above-described type is disclosed in U.S. Pat. No. 5,421,416, entitled "Hitch Assembly Control System", commonly assigned to Case Corp., which includes an electronic control unit for generating control signals applied to hydraulic valves configured to control the flow of hydraulic fluid to a hydraulic cylinder which raises and lowers a hitch assembly.
Work vehicles such as agricultural tractors may also be called upon to pull other types of implements throughout the field being worked, in some cases before or after the same vehicle has been used with a hitched implement. For example, implements known as "trailed implements" are typically pulled behind the work vehicle via a draw bar. Some trailed implements include their own hydraulic lift systems including one or more cylinders for raising and lowering components of the trailed implement. When pulling a trailed implement having its own hydraulic lift system, it is desirable to control the actions of the trailed implement lift cylinder as opposed to controlling the actions of the hitch assembly lift cylinder. This may be accomplished by providing a separate control unit and separate hydraulic valve on the vehicle which are dedicated to controlling the trailed implement lift cylinder. However, the need for separate control units for controlling the hitch assembly lift cylinder and the trailed implement lift cylinder, and for separate hydraulic valves, increases complexity and cost compared to a control system having a single control unit and a single hydraulic valve. Requiring separate devices for commanding the hitched and trailed implements also increases complexity and cost. Thus, it would be desirable to provide a control system for a work vehicle which includes a single control unit and single hydraulic valve for controlling different types of implements capable of being hitched to or trailed by the work vehicle, wherein control can be selectively transferred between a hitched implement or a trailed implement with a minimum of complexity and effort. It would also be desirable to provide the same type of control over a trailed implement as over a hitched implement, and to allow the same command devices to be used for commanding either type of implement.
In other situations, work vehicles may be called upon to move yet another type of implement. Implements of this type are also coupled to the hitch assembly, and can be raised and lowered in response to hitch control signals as described above. However, these implements also include hydraulically-powered gage or assist wheels to help support some of the implement's weight, thereby improving vehicle stability in the field and at headlands. The gage or assist wheels are powered by an implement hydraulic lift system which must also be controlled. It would also be desirable to provide a control system capable of simultaneously controlling the hitch assembly and such an independent implement lift system. It would further be desirable to provide such a control system which uses the existing auxiliary control system of the work vehicle to help provide these control functions.
There are also situations where an implement (e.g., a pesticide or herbicide tank) is attached to the work vehicle's hitch assembly, and it is desired to maintain the hitch assembly at a fixed position (e.g., at the uppermost position of the hitch assembly). In these situations, it would be desirable to provide a control system which controls the flow of hydraulic fluid directed to the implement while simultaneously insuring that the hitch assembly remains in the fixed position. It would also be desirable to provide such a control system wherein a low flow of fluid is provided to the actuator maintaining the fixed position of the hitch assembly for additional stability and to account for low-level leakage which would otherwise cause the hitch assembly to move from the fixed position (e.g., to be lowered).
Since a single work vehicle such as an agricultural tractor may be called upon to move any or all of these types of implements within a short period of time, there is a need for an adaptable control system for selectively controlling the operations of various types of implements which may be hitched to or trailed by a work vehicle such as an agricultural tractor. There is also a need for a control system for an implement hitched to or trailed by a work vehicle which selectively provides one of several different control functions by simply switching from one control function to another. A control system of this type provides the vehicle operator with additional flexibility in using the work vehicle, and provides the work vehicle manufacturer with diminished complexity and expense of manufacturing.